Blade for gas turbines with choke cross section at the trailing edge

ABSTRACT

In a gas-turbine guide element ( 30 ) around which a hot air flow ( 23 ) flows and which, at least in a trailing edge region ( 21 ), in which the air flow ( 23 ) separates from the guide element ( 30 ), comprises at least two walls ( 10, 11 ) arranged essentially in parallel and connected to one another by ribs ( 16, 17, 20 ) in such a way as to form internal cooling passages ( 18, 19, 25, 26, 27 ), and which is cooled on the inside with cooling medium ( 28, 29 ) flowing through the cooling passages ( 18, 19, 25, 26, 27 ), the cooling medium discharging from the guide element ( 30 ) at the trailing edge ( 21 ) essentially parallel to and between the walls ( 10, 11 ), and in a method of producing it, easier reworking and less susceptibility to foreign particles are achieved owing to the fact that at least some of the ribs are arranged as choke ribs ( 24 ) so as to terminate essentially flush with the trailing edge ( 21 ).

[0001] The present invention relates to the field of guide elements, such as guide or turbine blades, used in gas turbines. It concerns a gas-turbine guide element around which a hot air flow flows and which, at least in a trailing edge region, in which the air flow separates from the guide element, comprises at least two walls arranged essentially in parallel and connected to one another by ribs in such a way as to form internal cooling passages, and which is cooled on the inside with cooling medium flowing through the cooling passages, the cooling medium discharging from the guide element at the trailing edge essentially parallel to and between the walls.

[0002] A gas turbine comprises a multiplicity of elements which are subjected to a flow of hot working air. Since the working air is at a temperature which, for many of the materials from which such components around which flow occurs are constructed, leads to pronounced wear phenomena, in particular during a prolonged operating period, it is necessary to cool many of these components. In this case, the cooling may be designed as internal cooling, in which the elements are designed as hollow profiles or are simply provided with internal cooling passages through which a cooling-air flow is directed. Alternatively or in addition, it is also possible to provide so-called film cooling, in which a cooling-air film on the outside is applied to the elements.

[0003] Modern gas-turbine blades mostly use a combination of the above methods, i.e. an internal convective cooling system which additionally has openings for film blowing at critical points is used. In order to increase the efficiency and the output of the gas turbine, and in order to reduce the emissions, the quantity of cooling air used must be minimized. This means that only a small cooling-air mass flow is available even for large components. In order to realize and control the small cooling mass flows with efficient internal heat transfer, which is required at the same time, the cross sections of flow must be reduced accordingly, or choke cross sections must be introduced.

[0004] In many of the known blade designs, the choking of the cooling mass flow takes place in the region of the cast trailing blade edge, in the vicinity of the cooling-air outlet. In particular for production reasons, the end of the ribs which connect the pressure-side and suction-side walls is set back in the axial direction in order to avoid core fractures, i.e. the ribs already end in the interior of the blade and do not extend up to the trailing edge.

[0005]FIG. 1 shows a section through a guide blade according to the prior art, as often used in gas turbines. This is a section through a guide blade as typically used directly downstream of the combustion chamber and in front of the first moving row of the gas turbine for the optimum incident flow to the moving blades, the section running axially to the main axis of the turbine and perpendicularly to the blade-body plane. The blade is designed as a hollow profile, which is defined on the suction side by a wall 10 and on the pressure side by a further wall 11. In the incident-flow region, the blade is widened, the walls 10 and 11 are connected to one another in a rounded portion, and a central, radially running insert 12, around which the cooling passage leads, is located between the walls 10 and 11. In the rear region, the guide blade 30 is defined only by the two walls 10 and 11, and cooling passages run in between, the walls 10 and 11 being connected to one another by interrupted ribs running in the axial direction. The central insert 12 is often completely or partly enclosed by approximately axially running ribs. These ribs converge at the rear end of the insert (16 in FIG. 1) and from this point on connect the suction- and pressure-side blade walls. Approximately axial passages, in which the cooling air is directed, form between the ribs.

[0006] In its further course, the rib bank may be interrupted in order to produce a plenum 18 running in the radial direction. The following rib bank 17 may be arranged both in line with or offset from the previous rib bank. In the region of the trailing edge, the pressure- and suction-side walls are connected to one another by very short ribs or so-called pin rows. The prior art, then, is to allow these built-in components (ribs, pins, etc.) in the interior of the blade ends. This avoids the situation in which the core required for the casting production has a large jump in the cross-sectional area precisely at the trailing edge. This considerable nonuniformity in the core cross-sectional profile leads in fact to a high number of core fractures during production. However, the above method has the considerable disadvantage that the outlet cross section of the cooling air and thus of the cooling-air mass flow can be controlled only in an inadequate manner.

[0007] In addition, the walls usually have film-cooling holes 13-15, through which cooling air can flow to the outside.

[0008] This configuration of the internal convective cooling system has a number of disadvantages:

[0009] Since the cross section is small, even small tolerances during the production (casting) have an effect on the cooling-air mass rate of flow.

[0010] Since the choke point lies in the interior of the guide element, the effective choke cross section can only be measured and checked with difficulty.

[0011] Since the choke edge lies in the interior of the guide element, the effective choke cross section can be modified subsequently only with difficulty.

[0012] The two usually very thin walls are extremely susceptible to damage which is caused by foreign bodies in the hot gas and which may possibly even lead to a change in the choke cross sections.

[0013] Due to the gradual expansion of the cooling air (1) at the end of the ribs and (2) at the trailing blade edge, the cooling-air mass flow can be controlled and adjusted only with difficulty.

[0014] Accordingly, the object of the invention is to provide a gas-turbine guide element around which a hot air flow flows and which, at least in a trailing edge region, in which the air flow separates from the guide element, comprises at least two walls arranged essentially in parallel and connected to one another by ribs in such a way as to form internal cooling passages, and which is cooled on the inside with cooling medium flowing through the cooling passages, the cooling medium discharging from the guide element at the trailing edge essentially parallel to and between the walls.

[0015] This object is achieved in the case of a guide element of the type mentioned at the beginning by virtue of the fact that at least some of the ribs are arranged so as to terminate essentially flush with the trailing edge. The essence of the invention therefore consists in arranging some of the ribs connecting the walls directly at and essentially flush with the trailing edge and in thus making the ribs or the passages in between more accessible and in stabilizing the walls in the edge region more effectively. In this way, the walls in the trailing edge region are substantially less susceptible to damage caused by foreign bodies entrained in the working air flow. In addition, this also results in the advantage that the rate of flow of cooling medium through between the ribs arranged at the trailing edge can be reworked or adapted in a substantially simpler manner after the production process or during maintenance as a result of the good accessibility.

[0016] A first preferred embodiment of the invention is characterized in that the rate of flow of cooling medium through the guide element is essentially determined by the dimensioning of the outlet openings arranged between the ribs, here so-called choke ribs. The better accessibility and ease of reworking due to the arrangement are especially advantageous when the choking of the cooling-air circulation is effected by the choke ribs arranged at the trailing edge, and the choking can easily be set or even measured from outside by boring out or the like.

[0017] Another embodiment of the invention is characterized in that the thickness of the guide element at the trailing edge is within a range of 0.5 to 5 mm, in particular preferably within a range of 1.0 to 2.5 mm, and in that the slot thickness of the cooling-air passages between the walls at the outlet is within a range of 0.3 to 2 mm, in particular within a range of 0.8 to 1.5 mm. Inter alia, if the guide element is designed as a guide blade arranged in front of a turbine rotor, and if the cooling medium used is air, the arrangement according to the invention and this dimensioning prove to be especially advantageous.

[0018] Further embodiments of the guide element follow from the dependent claims.

[0019] Furthermore, the invention comprises a method of producing a gas-turbine guide element around which a hot air flow flows and which, at least in a trailing edge region, in which the air flow separates from the guide element, comprises at least two walls arranged essentially in parallel and connected to one another by ribs in such a way as to form internal cooling passages, and which is cooled on the inside with cooling medium flowing through the cooling passages, the cooling medium discharging from the guide element at the trailing edge essentially parallel to and between the walls, which method is distinguished in that the guide element is produced by a casting process, in that the trailing edge region is in this case cast with a projecting length extending the guide element or its walls in the direction of flow, and in that the projecting length is removed after the casting in such a way that at least some of the ribs are arranged as choke ribs so as to terminate essentially flush with the trailing edge. In this case, the casting core is formed in such a way that the rib geometry beyond the trailing edge of the blade is modeled in the casting core. The rib geometry is not blanked out until after a length of about 0.5 to 5 times, in particular 1 to 3 times, the core thickness. This method makes the simple production of a guide element according to the invention possible for the first time. This is because, in a normal casting process, the effective choke cross section cannot easily be placed directly at the outlet edge. The abrupt widening in the cross section at the outlet in the casting core leads to a considerable increase in core fractures during production. However, this can be avoided by leaving a projecting length during the casting process.

[0020] A preferred embodiment of the method is characterized in that no ribs are arranged between the walls in the region of the projecting length, and in that the rate of flow of cooling medium through the finished guide element is essentially determined by the dimensioning of the outlet openings arranged between the choke ribs. If ribs of any kind are dispensed with in the region of the projecting length, core fractures can largely be avoided during the casting process, in particular during the preferred pressure casting process (investment casting) . Furthermore, it is found that, in particular if the projecting length value is 0.5 to 3 times as large as the slot thickness, in particular preferably the same size as the slot thickness, of the cooling-air passage between the walls, such core fractures can be avoided without the need for excessive reworking after production.

[0021] Further preferred embodiments of the method follow from the dependent claims.

[0022] The invention is to be explained in more detail below with reference to exemplary embodiments in connection with the drawings, in which:

[0023]FIG. 1 shows a cross section through a guide blade with internal cooling for a gas turbine according to the prior art; and

[0024]FIG. 2 a) shows a cross section through a guide blade with choke ribs arranged directly at the trailing edge of the blade, b) a detail view of the trailing-edge region of the blade according to a), and c) a section along line X-X in FIG. 2a), i.e. essentially parallel to the plane of the blade through the internal cooling passage.

[0025]FIG. 2a) shows a section through a guide blade having ribs 24 directly adjacent to the trailing edge between the walls 10 and 11. This section through a guide blade is a section corresponding to FIG. 1 and running axially to the main axis of the turbine and perpendicularly to the blade-body plane. The blade is again designed as a hollow profile, which is defined on the suction side by a wall 10 and on the pressure side by a further wall 11. In the rear region, the guide blade is defined only by the two walls 10 and 11, and cooling passages run in between, the walls 10 and 11 being connected to one another by ribs interrupted in the radial direction. FIG. 2c) shows a section along line X-X in FIG. 2a), i.e. essentially parallel to the blade-body plane. First ribs 16 are located directly adjacent to the insert 12. The cooling air flowing between insert 12 and the walls 10 and 11 flows essentially axially in the passages 27 between the ribs 16 into the rear region of the guide blade. Located behind the first row of ribs 16 is a front radial plenum 18, which permits a flow and pressure balance of the cooling air in the radial direction. Adjoining the plenum 18 is a further row of ribs 17, which in this example are alternately designed as continuous ribs 17 b or as axially split ribs 17 a. The individual ribs of the rows 16 and 17 advantageously have a so-called spacing ratio, the ratio of the radial width e normal to the plane of the blade body to the radial spacing f, within a range of 0.25 to 0.75.

[0026] A further radial plenum 19 follows, followed by so-called pins 20, i.e. rows of ribs which are designed as simple webs and permit as uniform a distribution of the cooling-air flow as possible at the trailing edge 21. In this case, the spacing ratio (diameter g to radial spacing h) of the pins 20 lies within a range of 0.25 to 0.7.

[0027] A further row of ribs 24 is now located directly at the trailing edge so as to terminate flush with the latter. In this case, the row of rear ribs is dimensioned in such a way that the choking of the cooling-air flow of the entire effective cooling-passage cross section is effected by the passages 25 between the so-called choke ribs 24. Owing to the fact that the choking is effected at the trailing edge 21 and with such a row of choke ribs 24, a number of advantages are obtained:

[0028] The effective choke cross section can easily be measured at the outlet edge.

[0029] Only one choke point is obtained exactly at the trailing edge, instead of two choke points at the end of the ribs and the trailing edge.

[0030] Inaccuracies in the choke region which possibly arise during the casting process can easily be reworked, since the choke points are accessible from outside.

[0031] The choke cross section can easily be varied if required.

[0032] The arrangement of the ribs right at the end of the blade leads to increased stability of the separation edge, and thus foreign bodies in the working air flow can do less damage to the trailing edge and the cooling of the component cannot be impaired so very much by such deformations.

[0033] Such a blade is usually produced by a casting process, as a rule an investment casting process. In this casting process, however, the effective choke cross section cannot easily be placed directly at the outlet edge during production. The abrupt widening in the cross section at the outlet in the casting core leads to a considerable increase in core fractures during production. However, this can be avoided by leaving a projecting length during the casting process. In this case, the cooling geometry reproduced in the core is extended beyond the actual boundary of the component. FIG. 2b) shows the edge region of such an element extended beyond the trailing edge by the value b. Ribs are advantageously no longer arranged in the region of the projecting length. The transition from the choke geometry then does not coincide with the core mounting, but rather a transition from the choke geometry to a continuous radial passage first of all takes place inside the extended component, and this transition may then be used as core mounting without the risk of core fractures. Depending on the process, this transition may be designed in many different ways in an optimum manner for the core mounting, i.e. it is not necessary for the two walls to simply be extended evenly toward the rear as shown in FIG. 2b); for example, a gradual projecting expansion, or narrowing or thickening, of the walls in the region of the projecting length is also conceivable. The projecting geometry is reworked, i.e. removed, to the desired length of the trailing edge after the casting, so that the choke points coincide with the trailing edge. This may be done, for example, together with reworking which is normally subsequently necessary, such as electrical discharge machining and laser drilling of the film-cooling holes 13-15.

[0034] In the exemplary embodiment specified, the trailing edge usually has a thickness d within a range of 0.5 to 8 mm, preferably within a range of 1.0 to 2.5 mm. The slot thickness c of the cooling-air passage is usually within a range of 0.3 to 2.0 mm, preferably within a range of 0.8 to 1.5 mm. In order to be able to effectively avoid core fractures during the casting process, the projecting length b beyond the trailing edge, in particular in the case of the above dimensioning, should be 0.5 to 5 times, preferably 1 to 3 times, the length a of the choke ribs 24; it is especially advantageous if the projecting length b is the same as the length a of the choke ribs.

LIST OF DESIGNATIONS

[0035]10 Suction-side wall

[0036]11 Pressure-side wall

[0037]12 Insert or core

[0038]13 Suction-side film holes

[0039]14 Film holes at the leading edge

[0040]15 Pressure-side film holes

[0041]16 Ribs adjoining the insert

[0042]17 Intermediate ribs

[0043]18 Front radial plenum

[0044]19 Rear radial plenum

[0045]20 Pins

[0046]21 Trailing edge of the blade

[0047]22 Outlet opening at the trailing edge

[0048]23 Working air flow

[0049]24 Choke ribs at the trailing edge

[0050]25 Cooling-air outlet openings at the trailing edge

[0051]26 Axial passages between ribs 17

[0052]27 Axial passages between ribs 16

[0053]28 Inlet-side cooling-air flow

[0054]29 Outlet-side cooling-air flow

[0055]30 Guide blade

[0056] a Length of the choke ribs

[0057] b Projecting length after casting

[0058] c Slot thickness of the cooling-air passage at the outlet

[0059] d Thickness of the guide blade at the trailing edge

[0060] e Width of the choke ribs

[0061] f Rib spacing of the choke ribs

[0062] g Width of the pins 20

[0063] h Spacing of the pins 20 

1. A gas-turbine guide element (30) around which a hot air flow (23) flows and which, at least in a trailing edge region (21), in which the air flow (23) separates from the guide element (30), comprises at least two walls (10, 11) arranged essentially in parallel and connected to one another by ribs (16, 17, 20) in such a way as to form internal cooling passages (18, 19, 25, 26, 27), and which is cooled on the inside with cooling medium (28, 29) flowing through the cooling passages (18, 19, 25, 26, 27), the cooling medium discharging from the guide element (30) at the trailing edge (21) essentially parallel to and between the walls (10, 11), characterized in that at least some of the ribs (24) are arranged so as to terminate essentially flush with the trailing edge (21).
 2. The guide element (30) as claimed in claim 1 , characterized in that the rate of flow of cooling medium (28, 29) through the guide element (30) is essentially determined by the dimensioning of the outlet openings (25) arranged between the choke ribs (24).
 3. The guide element (30) as claimed in claim 2 , characterized in that the choke ribs (24) have a width (e) parallel to the trailing edge (21) and are arranged at a distance apart by in each case a rib spacing (f), and in that the ratio of width (e) to rib spacing (f) is within a range of 0.25 to 0.75.
 4. The guide element (30) as claimed in one of the preceding claims, characterized in that the thickness (d) of the guide element (30) at the trailing edge (21) is within a range of 0.5 to 5 mm, in particular preferably within a range of 1.0 to 2.5 mm, and in that the slot thickness (c) of the cooling-air passages (25) between the walls (10, 11) at the outlet (21) is within a range of 0.3 to 2 mm, in particular within a range of 0.8 to 1.5 mm.
 5. The guide element (30) as claimed in one of claims 1 to 4 , characterized in that it is designed as a guide blade (30) arranged in front of a turbine rotor, and in that the cooling medium used is air.
 6. The guide blade (30) as claimed in claim 5 , characterized in that the guide blade is designed to be widened in its incident-flow region, and in the incident-flow region the cooling air flows in suction-side and pressure-side cooling passages around an inner, central, radially running insert (12), and in that the cooling air flows through between ribs (16) adjoining the insert (12), then between intermediate ribs (17), then between pins (20) between the walls (10, 11) before it discharges from the guide blade (30) through outlet openings (25) at the trailing edge.
 7. A method of producing a gas-turbine guide element (30) around which a hot air flow (23) flows and which, at least in a trailing edge region (21), in which the air flow (23) separates from the guide element (30), comprises at least two walls (10, 11) arranged essentially in parallel and connected to one another by ribs (16, 17, 20) in such a way as to form internal cooling passages (18, 19, 25, 26, 27), and which is cooled on the inside with cooling medium (28, 29) flowing through the cooling passages (18, 19, 25, 26, 27), the cooling medium discharging from the guide element (30) at the trailing edge (21) essentially parallel to and between the walls (10, 11), characterized in that the guide element (30) is produced by a casting process, in that the trailing edge region (21) is in this case cast with a projecting length extending the guide element (30) or its walls (10, 11) in the direction of flow, and in that the projecting length is removed after the casting in such a way that at least some of the ribs are arranged as choke ribs (24) so as to terminate essentially flush with the trailing edge (21).
 8. The method as claimed in claim 7 , characterized in that the rate of flow of cooling medium (28, 29) through the finished guide element (30) is essentially determined by the dimensioning of the outlet openings (25) arranged between the choke ribs (24).
 9. The method as claimed in one of the preceding claims, characterized in that the casting process is a pressure casting process, in that the projecting length has a value (b) behind the trailing edge (21), in that the walls (10, 11) are kept at a distance apart at the outlet (21) by a slot thickness (c) of the cooling-air passages (25), and in that in particular the projecting length value (b) is 0.5 to 5 times, in particular preferably 1 to 3 times, as large as the slot thickness (c).
 10. The method as claimed in one of the preceding claims, characterized in that the choke ribs (24) have a width (e) parallel to the trailing edge (21) and are arranged at a distance apart by in each case a rib spacing (f), in that the ratio of width (e) to rib spacing (f) is within a range of 0.25 to 0.75, in that the thickness (d) of the guide element (30) at the trailing edge (21) is within a range of 0.5 to 5 mm, in particular preferably within a range of 1.0 to 2.5 mm, and in that the slot thickness (c) of the cooling-air passages (25) between the walls (10, 11) at the outlet (21) is within a range of 0.3 to 2 mm, in particular within a range of 0.8 to 1.5 mm.
 11. The method as claimed in one of the preceding claims, characterized in that the guide element is a guide blade (30) arranged in front of a turbine rotor, and in that the cooling medium used is air. 